1996 Pontiac Grand Am 2.4L Stumbling on Acceleration

1996 PONTIAC GRAND AM
136,000 MILES • 4 CYL • FWD • AUTOMATIC
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GEORGEETEN
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1996 Grand Am 2.4L. New crate engine installed about 15,000 miles ago. Ran great until few months ago. Here's the deal: When i first start the car, it starts fine and then stumbles a bit, especially for the first quarter mile or so. Runs ok, but at expressway speeds acts like it wants to stall then engine surges and runs okay again. Here's the parts i have replaced: O2 sens, fuel pump, fuel regulator and fuel filter. Chilton manual says to check fuel pressure at schrader valve, but on this car there is none. I assume it is fuel related and a pressure reading would help tell me what it is, but can't figure where and how to test the pressure on this car. BTW, reading from O2 sens is .9v steady (too rich) and i have checked all vacuum lines and fuel lines for kinking, etc. Checked TPS and it is reading fine as well. HELP!!!
Apr 30, 2009 at 6:27 PM
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RASMATAZ
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1996 Pontiac Grand Am 2.4L Stumbling on Acceleration

Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires
Apr 30, 2009 at 6:32 PM
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ATKAPARKING
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This should be an OBDII system-see if any codes have been stored, even if the check engine light never came on. Check vacuum readings, maf and map operation, inspect throttle plate for carbon. If o2 sensor is running constant .9v without fluctuation, verify sensor ground. Does this system have two o2 sensors, one pre-cat and one downstream?
Apr 30, 2009 at 6:45 PM
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